Caption: Pharmaceutical research. Researcher adjusting one of several round-bottomed flasks that contain freeze-dried samples of active substances found in mushrooms and bacteria. Many synthetic drugs are based upon naturally-occurring substances that are found to have medicinal properties. Extracting the active substances (enzymes) and freeze-drying them preserves them for future research. Freeze-drying is the process of drying under a vacuum at low temperatures. The low temperatures slow the chemical breakdown of the enzymes, as does the absence of water. The vacuum, as well as drying the samples, excludes the damaging oxidation that is caused by oxygen in the air.